In density measuring instruments, it is known in the prior art to use a mechanical oscillator, a torsion oscillator or the like which is designed as a vibrator, and which is operatively connected with an apparatus for measuring the frequency and/or the duration of the oscillation by means of electro magnetic, electro dynamic or piezo-electric transducers and the like. The determination of the density is then reduced to a measurement of the resonant frequency of the mechanical oscillator which is filled with the sample. The oscillator, most appropriately, is made of glass and is excited electronically to an undamped oscillation, the frequency of which is dependent upon the mass of the oscillator and therefore upon the density of the sample. In the case of precision measurements, the frequency determination is most favorably achieved by means of a digital counter in connection with a quartz clock.
In order to get a precise measurement in such systems it is known to provide a two-point temperature controller. In this two-point controller type of system two reference points are provided, an upper reference point and a lower reference point. When the temperature of the fluid rises above the upper reference point, the system is cooled to cause the temperature of the fluid to stay within the range between the upper and lower points. When the temperature goes below the lower reference point, heat is put into the system to bring the temperature back up between the upper and lower points. Thus, it may be seen that in such a two-point temperature controller the actual temperature may drift up and down between upper and lower points rather than staying at the desired level for density measurement.
A second problem with such two-point temperature controllers is the lag time between switching the direction of the heat flow and sensing the crossing of the set points. This time lag is due to the thermal time constant of the measuring cell. At high (37.degree. C.) and low (10.degree. C.) measuring temperatures this time lag causes very unsymmetrical excursions of the actual temperature with respect to the set point value.
Thus, it is an object of the present invention to provide a density measuring instrument in which the temperature does not drift up and down between two reference points and is held closely to a predetermined temperature value to permit accurate measurement of density.
It is further object of the present invention to eliminate the lag time between switching and sensing of the crossing of a reference point.